Dielectric relaxation studies of 6OCB/8OCB mixtures with the nematic-smectic A-nematic re-entrant phase sequence

The low frequency relaxation process was studied for 6OCB/8OCB mixtures wit h three concentrations (27.0, 27.3 and 27.5 wt %) exhibiting the isotropic- nematic-smectic A-nematic re-entrant-crystalline phase sequence and four mi xtures (28.5, 30.0, 35.0 and 40.0 wt %) with the isotropic-nematic-crystall ine phase sequence. In the liquid crystalline phases, all dielectric spectr a could be excellently described by the Debye equation. The relaxation time tau (parallel to) passes smoothly through the phase transitions separating the liquid crystalline phases. The activation barriers hindering the molec ular rotations around the short axes are practically the same in the nemati c and smectic phases and become larger in the re-entrant nematic phase. Sma ller values of the barrier in the nematic phase of mixtures in comparison w ith those obtained recently for pure 6OCB and 8OCB are explained as an effe ct of weakening of the molecular interactions caused by increased dipole-di pole associations between molecules in mixtures in relation to pure substan ces. The slightly larger activation barrier in the nematic re-entrant phase indicates stronger molecular associations in this phase.